Introduction (relevance): one of the requirements imposed by industry regulations governing the issues of internal control over the quality and safety of medical activities is the mandatory accounting and analysis of adverse events occurring in a medical organization. The experience of interaction with medical workers of city and district public health care institutions shows that there is a lack of understanding of these processes, both at the managerial level and at the level of practical implementation. The situation is further complicated by the fact that the procedure of accounting for undesirable events in our country is not defined, so the heads of medical organizations independently devise mechanisms for implementing the above requirements, which leads to different interpretations not only of the term "undesirable event" itself, but also of the process of its identification. The aim of this article is to study the essence of the term "undesirable event" and to identify the types of undesirable events that may occur in a medical organization. Materials and methods: the methods of content analysis were used, publications of Russian and foreign authors, placed in scientific periodicals, including in RINC (eLIBRARY.RU) and CyberLeninka databases, sectoral normative-legal acts, national and international standards were studied. Results: in most existing regulatory documents and standards the essence of the term "undesirable event" is based on the presence of influence of some facts and circumstances on the patient. In national standards defining requirements for system management (quality management, risk management), the term "undesirable event" implies the occurrence of changes in conditions and circumstances that led to the failure of the organization or its individual processes. And only in the sectoral order the term "undesirable event" implies the presence of facts and circumstances that can cause harm to both the patient and the health care worker, excluding that such an event can also happen to non-medical personnel. Conclusions: none of the considered definitions fully reflects the essence of the term "undesirable event", which allows medical organizations to interpret it differently and, accordingly, affects the general understanding of the mechanism of identification and accounting of undesirable events. In our opinion, the most accurate definition of the term "undesirable event" is the following: an undesirable event is an event resulting from a change in circumstances and conditions of activity, resulting in causing (threatening to cause) harm to a patient (affect the timing of treatment) or employee, or negative changes in the activities of the medical organization. According to the definition, such events may include undesirable events related to the use of medicines, handling of medical devices, provision of medical care, violation of information security, professional activities of employees, violation of security of the organization's environment, organization and administration of processes, communication, violation of medical ethics and deontology, as well as infectious diseases related to the provision of medical care. Accounting of some of them is regulated by separate regulatory legal acts, national and industry standards, so it is advisable to develop a unified mechanism for their registration and accounting in a medical organization.